How to use a wildcard-containing SUMIF function in VBA?

I would like to put the following functions in the cells of a given column if someone clicks on the button of a userform:

=IF(A3<>"";SUMIF(Items!$A:$A;"*"&Summary!$A:$A&"*";Items!$E:$E);"")

Should I go for an If and Then structure or create a user defined function?

And how could I express that (especially the wildcard part)?

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